(1) Field of the Invention
The present invention relates to a manufacturing method of an arc tube having a glass tube of a flat-spiral shape, the arc tube, and a fluorescent lamp including the arc tube.
(2) Related Art
Recently in the lighting field, various types of compact self-ballasted fluorescent lamps (hereinafter, simply “fluorescent lamp”) are being developed as energy/resource saving light sources. One example thereof, though not yet in the actual use, is a fluorescent lamp having an arc tube shaped like a disc whose discharge path has a double flat-spiral configuration. This type of fluorescent lamp is hereinafter referred to as “flat-spiral fluorescent lamp”. Such a flat-spiral fluorescent lamp is small in size compared to a conventional circular fluorescent lamp, and has characteristic advantages such as favorable luminous intensity distribution capable of realizing a round light-emission surface.
According to a conventional manufacturing method of flat-spiral fluorescent lamp, the process of forming a glass tube into a flat-spiral shape is performed as follows. A jig having a substantially circular cone shape is prepared. A softened straight glass tube is wound along a conical surface of the jig into a double spiral configuration, thereby forming an intermediate body having a substantially circular cone shape (for actual methods, please refer to West German Patent No. 860675 and West German Patent No. 871927, for example). Next, The thus-formed intermediate body is shaped into a disc shape by being pressed under a temperature of or below a softening point, thereby completing the glass tube into a flat-spiral shape (hereinafter “flat-spiral glass tube”).
Meanwhile, as shown in FIG. 1, so as to obtain a small flat-spiral fluorescent lamp and to reduce luminance inconsistency of the light-emission surface at the same time, it is preferable that the distance Gb between adjacent exterior walls of a flat-spiral portion 510 of a flat-spiral glass tube 500 is uniform and short. In view of this, in the mentioned process of forming a flat-spiral glass tube, a straight glass tube is wound to a jig in a constant and comparatively short winding pitch.
In addition, it is required to keep a distance Ge between the end portion 520a (520b) and the flat-spiral portion 510 to a predetermined length or more, so as to prevent softening/deformation of the flat-spiral portion 510 adjacent to the end portion 520a (520b) attributable to heat applied by a burner or the like in the electrodes-providing process within end portions 520a and 520b. Furthermore, the distance Ge is required to be at a predetermined length or more also for the purpose of attaching bases to the end portions 520a and 520b. (Although it is possible to prevent the above-mentioned softening/deformation of the flat-spiral portion 510, by decreasing the thermal power of the burner or the like, the insufficient heating may-cause sealing failure such as crack, leak and the like at the sealed portion, leading to yield factor reduction for the electrodes-providing process.)
In the stated conventional method, after the process of forming the flat-spiral glass tube, re-processing is performed to heat and soften predetermined areas of the flat-spiral portion 510 near the end portions 520a and 520b using the burners 530a and 530b for example, and to bend the predetermined areas outward so that the end portions 520a and 520b are moved to positions shown by solid lines from positions shown by imaginary lines. As a result of this re-processing, the distance Ge is increased.
However, the re-processing necessitates a process that includes heating, forming, and cooling, which takes a comparatively long time. Consequently, the manufacturing cost of the fluorescent lamp will increase. In addition, after the re-processing, the flat-spiral glass tube 500 will be deformed from the smooth double flat-spiral configuration realized using the jig. This impairs the appearance of the fluorescent lamp.